The invention relates to liquid crystal pigments, to methods of their preparation and to their use in reflective films or coatings, for decorative purposes, cosmetic and pharmaceutical compositions, optical elements and security applications.
In prior art platelet shaped liquid crystal pigments are known which comprise a layer of prealigned polymerized cholesteric liquid crystal (CLC) material, wherein the cholesteric helix is oriented perpendicular to the plane of the layer. These pigments show selective reflection of visible light that is circularly polarized, caused by interaction of incident light with the helically twisted structure of the CLC material. The central wavelength of reflection xcex depends on the pitch p and the average refractive index n of the CLC material according to the equation
xcex=nxc2x7p 
Due to the platelet shape of the pigments, the effective helical pitch of the CLC layer that interacts with incident light, and thereby the wavelength of the reflected light, vary depending on the viewing angle. This results in a colour flop of the reflection colour of the pigment when being viewed at different angles. This angle dependent colour effect is exploited in many applications, such as decorative uses like inks or lacquers e.g. for car bodies, or security uses like non-forgeable security markings for documents of value.
Platelet shaped CLC pigments are typically produced by coating a thin layer of a polymerizable or crosslinkable CLC material onto a substrate, aligning the CLC material to achieve uniform orientation of the helical axis perpendicular to the plane of the layer, and curing the CLC material for example by UV irradiation. The cured film is then ground to yield small platelet shaped flakes.
For practical use in inks or lacquers, the CLC pigments are usually dispersed in a light transmissive binder. The inks or lacquers can then be used at room temperature without the need of further alignment.
Platelet shaped CLC pigments with viewing angle dependent colour as described above are disclosed for example in EP 0 601 483 and WO 97/27252.
The pigments disclosed in the above mentioned documents, however, reflect only a narrow waveband of light. Therefore, in order to obtain inks or lacquers with mixed or white colour characteristics which are desired for specific applications, it is necessary to mix together pigments with different primary colours which have to be fabricated separately.
A more time and cost effective way would be to use LC pigments that directly show mixed or white colour characteristics. This can be achieved by providing LC pigments with a broadened bandwidth of the reflected wavelength band, which covers a substantial part of the visible spectrum or, in case of white colour, even covering the entire visible spectrum.
WO 99/02340 discloses cholesteric pigment flakes with a broadened bandwidth of the reflected waveband. These pigments comprise a bilayer of polymerized CLC material. Each layer exhibits a non-linear pitch distribution throughout its thickness direction, and as a result shows reflection of a broad wavelength band with asymmetrical reflection characteristics. The pitch gradient is achieved by mixing together a cholesteric polymerizable or crosslinkable liquid crystal material and a nematic non-polymerizable liquid crystal material, coating the mixture onto a substrate, annealing the mixture to achieve planar alignment and curing the mixture e.g. by UV irradiation. Upon curing of the mixture phase segregation of the non-polymerizable nematic material and the polymerized cholesteric material occurs, leading to a pitch distribution, including regions with high pitch and regions with low pitch, within the material.
The pigments according to WO 99/02340 are obtained by laminating together two similar single layers of CLC material with non-linear pitch distribution and asymmetrical reflection characteristics in such a manner that the resulting bilayer has symmetrical reflection characteristics. This is achieved by laminating the two single layers together such that their surfaces with similar reflection characteristics are facing each other.
The pigments according to WO 99/02340, however, have several drawbacks. Thus, the preparation of the single CLC layers with non-linear pitch distribution is complicated and time consuming, since it relies on a phase segregation process which involves material diffusion within the CLC layer during polymerization. Furthermore, due to the bilayer structure of the CLC pigments their manufacture is complicated because an extra lamination step is needed. Furthermore, the resulting pigments have a high thickness, which is disadvantageous for many applications where thin pigments or coatings are required.